In constructing a pleated-membrane fluid flow transfer device, the outermost flaps of the pleated membrane have been sealed inside the apparatus housing, which also serves as the fluid chamber where fluids are passed on both sides of the membrane. One such construction is shown in Markley U.S. Pat. No. 3,788,482. It would be desirable from a manufacturing standpoint to do the sealing of the flaps in an area separate from the fluid flow chamber, as well as desirable to keep unnecessary potting out of the fluid flow chamber in order to minimize chance of blockage of flow passages.